1. Field of the Invention
The present disclosure relates to an optical scanning apparatus and an image forming apparatus including the optical scanning apparatus.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic method, such as a laser beam printer or a copying machine, includes an optical scanning apparatus which emits a light beam for exposing a photosensitive member. The image forming apparatus forms an electrostatic latent image on the photosensitive member using the light beam emitted from the optical scanning apparatus. The image forming apparatus then develops the electrostatic latent image using toner and forms an image.
FIG. 13 is a perspective view illustrating the optical scanning apparatus. Referring to FIG. 13, the light beam emitted from a light source 1301 is deflected by a reflection surface of a polygon mirror 1302. The light beam deflected by the polygon mirror 1302 then passes through fθ lenses 1303 and 1304, is reflected by a reflection mirror 1305, and reaches a surface of the photosensitive member. Optical members such as the polygon mirror 1302, the fθ lenses 1303 and 1304, and the reflection mirror 1305 are mounted on a housing 1306 of the optical scanning apparatus.
When the image forming apparatus forms an image, the polygon mirror 1302 is rotationally driven by a drive motor. In general, the drive motor rotates at high speed, i.e., at 20,000 rpm to 40,000 rpm, so that temperature of the drive motor rises by 15° C. or more after a few minutes from starting to be driven. Temperature distribution is thus generated inside the housing 1306 due to heat generated by the drive motor becoming driven. The heat distribution causes uneven deformation of the housing 1306 to generate distortion.
In particular, since an amount of rise in temperature is greater in a portion where the drive motor is disposed as compared to other portions, an amount of deformation of the portion where the drive motor is disposed becomes relatively larger. As a result, the housing 1306 sags at a bottom to be basin-like shape with the drive motor as a center, as illustrated in FIG. 14. Referring to FIG. 14, the amount of distortion is exaggerated as compared to an actual amount of distortion so that deformation is easily recognizable. Since the deformation changes orientation of the optical members mounted on the housing 1306 from the desired orientation, an optical path of the light beam is changed from the desired optical path. Quality of the formed image is thus deteriorated.
In response to the above problem, Japanese Patent Application Laid-Open No. 2009-198890 discusses an optical scanning apparatus in which an opening is formed in the vicinity of the drive motor to allow the housing to be capable of ventilation between inside and outside the housing. By forming an opening, the heat inside the housing is released, so that deformation of the housing can be suppressed.
The optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2009-198890 is capable of reducing heat deformation by forming the opening. However, strength (i.e., rigidity) of a peripheral portion of the opening is lowered by forming the opening.
Referring to FIG. 13, ribs (e.g., ribs 1307) which are reinforcement members are disposed in the housing of the optical scanning apparatus to increase the rigidity. Since the ribs are not disposed in a periphery of the opening in the optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2009-198890, the rigidity of the periphery of the opening is reduced.